The Government is facing increasing demands to publish a report into the running of Southern Railway amid speculation that it has highlighted an urgent need for safety improvements at one of the country's busiest stations.
The report by Chris Gibb, a non-executive Network Rail director with 35 years' experience in the industry, was handed to the Department for Transport last year.
Unions, passenger groups and politicians have been calling for the report to be published amid continuing problems on Southern, including the threat of further industrial action over staffing and driver-only trains.
The report is expected to highlight problems of passenger congestion at Victoria station in London, where many of Southern's services start or finish, according to sources.
It is understood that senior managers told Mr Gibb about the issue when he visited the station as part of his inquiry.
It is believed he was told there were not enough ticket gates for passengers using some of the busiest platforms, with long queues at peak times, and travellers complaining of chaotic scenes.
It is also understood that Mr Gibb was told that platforms dedicated for Gatwick Express trains were being routinely used by passengers from other busy trains.
For tourists travelling from Gatwick Airport, one of the first sights of London is of packed platforms and a wait to get through ticket barriers, Mr Gibb was reportedly told.
Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "It is scandalous that the Gibb report remains under lock and key in a vault at the Department for Transport.
"RMT is demanding its immediate publication as the stench of the Southern Rail fiasco hangs like a cloud over this rotten Government and reminds passengers that the alternative option of public ownership is now right up the agenda.
"We were told that the report would be published 'after the election'. Well, the election has been and gone and the Tories took a hammering along the length of the Southern routes as the electorate sent out the clearest possible message about their failing transport services.
"Not only does RMT want to see the Gibb report but the union is also stepping up the campaign to reverse the attack on the guards and to end the privatised rip-off that blights Britain's railways."
A spokesman for the Association of British Commuters said : "We are convinced that the Gibb report is extremely critical of the Department for Transport's role in a rail crisis that has always been about far more than just the strikes.
"There can be no other explanation for withholding such a vital report for almost six months, when its contents are so clearly in the public interest."
A DfT spokesperson said: “The Transport Secretary appointed Chris Gibb to lead a project board to improve Southern services. We have received his report and are committed to publishing it in due course.
“Performance on Southern has been consistently better since the new year and making sure passengers keep seeing a reliable and efficient service is a priority for the Government and the operator.
“We are investing £300m to improve performance and resilience on Southern’s route and nearly 60,000 passengers have now taken up a special package of compensation. Passengers on Southern are also the first in the country to benefit from compensation if their train is more than 15 minutes late.”